Effect of Ambient Conditions on the Resistance of Metal Oxides as a Novel Material for Outer Corona Protection Systems

Antimony doped tin oxide, a known gas sensing material, is investigated as a possible substitute for carbon black as filler for outer corona protection systems. Therefore it is necessary to prove the stability of electrical resistance and reversibility during exposure to discharge products like UV-radiation, ozone and NO
₂
. Furthermore, the location of air-cooled generators can vary and with it the surrounding atmosphere, for example humidity. Samples of particle filled resin films are prepared and the resistance is measured while exposed to different diluted gases (varying O
₂
, H
₂
, humidity and NO
₂
in the gas atmosphere at 100 °C). The resistance changes with varying O
₂
(%-range) and NO
₂
(ppm-range) concentrations. These effects are enhanced by UV-radiation (365 nm). All measured resistance changes are reversible but there is a long relaxation time. Besides, both the matrix material and the particle geometry of the Sb-doped SnO
₂
were changed. It was found that the resistance stability depends on the combination of matrix material and particle geometry. We found a link between resistance change and surface roughness, whereby a smoother surface seems to be more stable. The influence of ozone on the film resistance was also measured: Small amounts of ozone (<; 1 ppm) do not change the resistance of a compound film during ten days at room temperature. Hence, it is concluded that antimony-doped-SnO
₂
-compound-films are stable against different gas species and UV-light. However, it is important to choose a proper particle-resin combination for a smooth surface resulting in improved resistance stability.